Köhler Illumination

This setup, invented in 1893 by August Köhler, is very useful if you are dealing with any type of microscopy. It provides a homogeneous illumination on a sample, from any inhomogeneous light source. The image of the illumination source (e.g. lamp filament) is therefore not visible on the sample. The two apertures can be used to either change the intensity and numerical aperture (NA) or the field of view of your illumination. The second aperture is particularly advantageous, as you can close it to focus your sample. If the edges of the aperture are in focus, then the sample is focused too. It is also referred to as a 4f-setup, since it uses four focal lengths in distance.